Chapter 20: DRIVING VISION
"eyes don't tell people what to see..........people tell eyes what to look for"
Studies suggest up to 80% of all crashes can be avoided with better vision skills.
Our eyes are one of the most important safety features we have when driving. They allow us to position the vehicle, gauge our speed, see potential hazards and manoeuvre the vehicle.
About 90% of the information we use when driving is gathered by our vision. Other senses such as hearing, touch, feel, balance and smell provide the remaining information.
It is concerning then that Australian transport departments no longer test visual acuity when renewing licences.
Unfortunately we over-rely on one sense, a sense that does have limitations. The human body has never evolved to be able to drive a car. As such we still have the eyes and other senses of primitive man. The following sections outline the different limitations of human vision:
We have the widest range of vision (largest visual field) when stationary. Typically our primary vision is within 10° - 12° of our line of sight. But objects can be detected outside of this in our peripheral vision, 90° to the left and right, 60° above and 70° below our line of sight. This means when stationary we have a 180° horizontal visual field and a 130° vertical visual field.
At speed however our vision focuses automatically further ahead and thus the visual field reduces in all directions. At 100 km/h our horizontal visual field is only 40° (20° to the left and right of our line of sight).
Outside of the visual field we need stimulus to make us turn our head and look in that direction. A good stimulus could be movement relative to the background or a flashing or pulsating light or some other bright light.
This is one reason why our indicator lights flash, so they can be noticed and seen by other drivers even outside of their field of vision.
This is also a good case for the use of daytime running lights on vehicles, especially when driving at high speeds to make sure your being seen by other drivers.
RAPID EYE MOVEMENT (REM)
The maximum movement rate for a human eye is 4 fixations per second and can only be sustained for a short term.
An alert, busy and skilled driver, giving their full attention to driving is capable of 2 fixations per second. This information then must be processed.
Therefore at 100km/h (28m/s) an alert driver is not physically capable of seeing something, comprehending it and making an appropriate response or decision unless the hazards/objects are spaced at least 20 metres apart. Inputs received faster than this lead to information overload and concentration is diverted from driving to process this additional information.
Under normal driving conditions most drivers achieve 1-1.5 fixations per second whilst;
- talking on the mobile phone and/or
- talking to passengers and/or
- tuning the radio, changing cassettes or compact discs and/or
- not watching where they are going.
If you are like me, at some time you have driven through an intersection and then asked yourself, "Gee I hope that light was green", a quick check in the rear vision mirror confirms if vehicles behind followed you through the intersection. What is happening is our mind is processing other information instead of concentrating on the driving task, eye fixations and comprehension of what we see has been reduced. Driving in a daydream!
RANGE OF EYE MOVEMENT
With our head stationary our eyes can scan 50° of our visual field.
However as we have evolved from hunter gatherers with technology, we don't utilize this range of eye movement.
Instead we turn our heads to focus in other directions. As such we limit eye movement to only 15° left and right.
Only leading sports professionals have trained themselves to better use their eyes and their visual field.
The human visual system is capable of operating over an enormous range of illumination, due to 2 factors;
The pupil of the eye can contract or dilate to let in more or less light.
After a period of darkness, receptor cells in the retina of the eye begin to regenerate. As such after 30 minutes of darkness our night vision can improve up to 10,000,000 times.
The pupil's rapid contraction or dilation maintains vision in sudden change in light condition. However the pupil's diameter can reduce (contract) 6 times faster to sudden brightness after darkness, than exposure to darkness after brightness.
As such entry to an undercover carpark, building, garage, tunnel or canopy of trees should be done with extreme caution since their is a period of reduced vision as our eyes adjust to the darkness.
Glare from headlights or street lights can also reduce our vision. When driving a night care must be taken to ensure headlights are not left on high beam when a vehicle approaches, as you are somewhat blinding the driver.
If a vehicle approaches you with their high beam on, look to the edge of the road on your side, to reduce the glare until they have passed. If they are approaching from a long way off, a brief flash of your high beam might notify them of their mistake. Remember flashing your lights at them constantly is only blinding them.
Velocitation is caused by long periods of high speed travel. The eyes become fatigued in the horizontal plane to images streaming through the windscreen. Velocitation can be noticed when entering built-up areas after long periods of country driving. The speed drop from 100 km/hr to 60 km/hr makes the driver think the car is going much slower (maybe walking pace). The eyes can no longer judge horizontal velocity correctly and as such cannot judge safe following distances. The only remedy is to stop the images streaming through the windscreen, to do this stop and park for several minutes to allow the eyes to rest.
It is for this reason, a driver on the highway may catch up to you and then follow close behind making no attempt to pass. The driver subconsciously has found it very relaxing to stare at the rear of your vehicle rather than scan the road ahead. In this situation you are now essentially steering both cars.
Your safest option would be to slow gradually and force the other driver to pass. Also check that you are not lowering your concentration by staring aimlessly at the vehicle in front of you.
VEHICLE BLIND SPOTS
The blindspots in a hatchback are very small, its the blind spots the driver has that are the big worry. The cars mirrors should be checked and adjusted correctly before driving. Unfortunately you see many drivers on the road with mirrors missing or aiming down at the road.
Its another sign of a driver ignorant to the risks, who persists in driving even though the passengers mirror is poorly aligned. Would you make the effort to stop and adjust the mirror or just take the attitude "Itll be all right".
Lets take a look at the blindspots a driver has.
With the drivers head and eyes positioned straight ahead, none of the sections are visible in the drivers vision.
Firstly there is a small blindspot created by both A-pillars, for a tall driver the rear-view mirror can create a distant blind-spot.
Outside of the drivers peripheral vision a blindspot extends around to what the side mirrors cover. This area is larger on the passengers side. These limitations of human vision create blindspots large enough to park a truck in.
As such it is imperative to adjust your mirrors correctly. Adjust the middle rear vision mirror so that from your correctly adjusted seating position you just cannot see the top of rear windscreen in the top of the mirror. Aim this mirror up from the road immediately behind your car.
Dont be afraid to USE your centre rear vision mirror, the mirror may also have an anti-glare adjustment for night driving.
Some drivers sub-consciously believe it is a sign of weakness to acknowledge the car behind or worse still make eye contact with the following driver. These are the drivers who daily fight for every bit of bitumen they can and will try anything not let another car pass. Using your mirrors keeps you aware of those fools around you; allowing you time to make the right decision when required.
In a car adjust the electric side mirrors out, so that the inside of the mirror just cannot see the side of your vehicle. Dont waste mirror by looking at the side of your car, it wont fall off and know one should ever overtake you that close.
Now there will always still be blindspots, so be prepared to look around the A-pillar when cornering and shoulder check blindspots before leaving a parking spot, indicating, merging and change lanes. Look out too for motorcyclists in your blindspots.
(Written by Joel Neilsen, Managing Director, Safe Drive Training)